Although electronic devices such as mobile phones and tablet computers have been widely used, many people still keep a habit of writing. This is due to a fact that the experience of writing on paper is irreplaceable by a touch screen and a handwriting tablet. In addition, the drawings and the like can be modified and annotated conveniently when writing on papers. However, storage of paper documents needs to occupy a large amount of space and it is not easy to carry and transport the paper documents. Therefore, electronization of texts and images written on papers is an urgent problem to solve.